Slap on a cap

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1. Sit on the edge of the pool and fill your swim cap with water. Hold it above your own head with two hands, or get someone else to do it for you. Note – it doesn't work with those less flexible fancy pants silicone swimming caps.

2. I got my dad to drop the cap on the count of three. Here it is accelerating towards my cranium at 9.8 metres per second per second!

3. Bullseye dad… right on my noggin! All the water is still inside the cap at this stage and it looks a bit like 'flubber'.

4. Thanks to its incredible surface tension and low viscosity, the water flows around the side of my head and turns the cap inside out.

5. A split second later, water is bursting out in every direction in a spectacular explosion. Water really is impressive stuff.

6. Ta-da! You can't see a thing of course and you look ridiculous but who cares? It feels quite amazing and is completely hilarious.

What's going on?

Most kids who attend swim squad training master this nifty trick at an early age and it appears to be fairly self-explanatory. But dig a little deeper and you'll discover a stack of science here, most of which physicists can't yet explain.

That might sound a bit sensationalist, but it's actually an understatement. Kids master the driving force behind this trick before they can even walk. We call it gravity but few people know it is one of just four fundamental forces which gave rise to the universe as we know it, and everything in it. In case you're wondering, the other three are the electromagnetic force, the strong nuclear force and the weak nuclear force.

Babies take great delight in dropping toys, cutlery and food from their high chairs for adults to retrieve. We all did this as infants and while it looks like 'play', psychologists call it learning. Predicting how objects respond to and move in this amazing invisible force field is a vital stage in every child's early development. You and I mastered gravity by the time we could walk. Now that you can read, it probably seems surprising that nobody knows why it exists.

Grown up physicists have been wondering about this since the dawn of thought and they're equally perplexed by the dawn of time (What is it? Why does it only move in one direction, if that is actually the case?). Centuries of these thoughts have led to the construction of the world's biggest machine - the Large Hadron Collider (LHC) at CERN in Geneva. Here, at last, they hope to get a glimpse of whatever it is that makes things like swimming caps full of water fall to the ground. In the process, they might even get a handle on time and come to grips with space. What we'll be able to do with that knowledge is anyone's guess.

When I sit wondering about all this, I find it equally amazing that anyone can wander around in this invisible force field and not become obsessed by finding out why it exists. But that's just me.

So yes, it's just gravity and the amazing properties of water and latex that make this trick work, but there's nothing simple about it.